Conventional cooling towers of the counterflow type employ a generally horizontal fill with an air opening below the lower surface of the same. Counterflow fills of the film type have a relatively good heat transfer coefficient. The air is drawn from below the fill and out the tower by a fan, or the draft from a high stack, positioned above the fill. When the distance between the fill and base of the tower is relatively small, the air must be drawn from the surrounding into the tower at a relatively high velocity and, when it reaches a position below the fill, it is forced to turn abruptly at a sharp angle to proceed upwardly through the fill. This requires high fan power requirements. On the other hand, by building the tower of relatively high supporting columns, the velocity of the incoming air is somewhat reduced but the overall height of the tower is substantially increased. Among the disadvantages of such height increases are increased pumping head, structural wind loads, and general appearance.
Conventional crossflow towers comprise a relatively thin vertical fill section with the water being fed from an overhead source and the air being drawn therethrough from air inlets at the side of the tower. Since there is no necessity for the air to make radical changes of direction in the fill and the air inlet is spaced along the entire height of the fill, the overall air pressure losses are usually less than those of a conventional counterflow tower as set forth above.
A crossflow cooling tower is inherently less efficient with respect to heat transfer than a counterflow tower based on a unit of fill. Another disadvantage of the crossflow cooling tower is that the water is loaded onto the top of the relatively thin crossflow fill. There is a maximum water load beyond which the water will not redistribute effectively because it will start gushing in a steady stream through the tower. When this maximum water load is exceeded in a crossflow tower of the film fill type, the water will not cling to the fill, leading to relatively poor heat transfer between the air and water. Also, resistance to the transversely flowing air is substantially increased requiring excessive fan power. This problem of water loading cannot be effectively overcome by widening the fill in the direction of air flow because there is a limiting factor on cooling efficiency relative to the thickness of the fill. A major factor in this limit is that the fan power for the longer air path through the fill disproportionately increases in comparison to the advantages to be attained by easing the above water load problems.
Corrugated film type fill is relatively efficient in either a counterflow or crossflow cooling tower. When utilized in conventional (horizontal) counterflow type tower, variations in the direction of the corrugations will affect the ease or difficulty of gas and liquid passage in a similar manner. For example, by disposing the corrugations at a relatively vertical inclination, the gas path is eased for lower power requirements but so is the liquid path leading to relatively poor film formation and low liquid residence time. On the other hand, by disposing the corrugations at a relatively horizontal inclination, the gas must travel through a tortuous path which greatly increases the fan power requirements.
U.S. Pat. No. 3,917,764 discloses a liquid-gas cooling tower which combines advantages of the counterflow and crossflow cooling towers. Specifically, that patent describes a cooling tower with a film fill section having an incline principal plane formed of a number of sheets mounted for the passage of gas and liquid. This sloping film fill section spreads the liquid gravitating onto its upper surface into a thinner, more uniform film on the lower surface. Splash-type fill is disposed inboard and/or outboard of the sloping fill. Corrugated and other types of film fill are disclosed. While this tower provides good contact characteristics, it is relatively expensive to install the splash fill. In the sentence bridging columns 9 and 10 of that patent, there is a suggestion of using widely spaced film fill functioning in the manner of splash fill in combination with the sloped film fill. One use of widely spaced film fill in a splash fill mode is disclosed in U.S. Pat. No. 3,758,088 in which members formed from corrugated boards are disposed on edge and spaced apart in a grid.
U.S. Pat. No. 3,450,393 discloses a crossflow cooling tower including a stack of two film fill sections with a thin high density fill disposed over a low density film fill section which performs the cooling. The upper fill section is used to provide an even diffusion of water falling from the distribution pan. The corrugated sheets are perpendicular to the direction of air flow through the lower section and so block the passage of air. At column 2, lines 63-69, the patent specifies that the air does not pass through the channels of this high density film.
U.S. Pat. No. 4,460,521 discloses another use of fills of different density, in this instance, all of the splash fill type in a crossflow cooling tower. In one embodiment, a sloped section of high density splash fill is flanked by triangular sections of low density splash fill.
Another combined fill application is disclosed in U.S. Pat. No. 4,317,785. That patent describes a cooling tower with a number of film fill box-like sections arranged in a stair-step configuration progressing with the highest section at the outboard end of the fill area and the lowest section at the inboard end. The remainder of the tower available for water distribution is filled with splash fill. Air travels horizontally through the film fill boxes.
U.S. Pat. No. 4,592,877 discloses a combination of crossflow and counterflow. FIG. 3 of that patent discloses two sloping film fill sections of the type described in U.S. Pat. No. 3,917,764 flanking a horizontal film fill section, all disposed above splash fill sloping generally inboard to define a partially open chamber.